This invention relates to surgical implants that are designed to replace meniscal tissue and possibly cartilage in a mammalian joint, such as a knee.
The structure and components of soft tissues are discussed in nearly any textbook on human physiology (e.g., Guyton and Hall, Textbook of Medical Physiology, 9th edition (1996) at page 186). Very briefly, the cells in most types of xe2x80x9csoft tissuexe2x80x9d (excluding bones, teeth, fingernails, etc.) are held together by a matrix (i.e., a three-dimensional network) of two types of fibers. One type is composed mainly of collagen, a fibrous protein that provides most of the tensile strength of tissue.
The other type of fiber consists mainly of xe2x80x9cproteoglycan filamentsxe2x80x9d. These filaments contain a small quantity of protein and a much larger amount (roughly 98%) of hyaluronic acid, a natural polymer with alternating saccharide rings of glucosamine and glucuronate. Unlike collagen fibers, which are thick and provide high levels of tensile strength, proteoglycan filaments are extremely thin, and cannot be seen under light microscopes. They cause the watery extra-cellular fluid in soft tissue to form a gel-like material called xe2x80x9ctissue gelxe2x80x9d. This gel contains water, the proteoglycan filaments, and any other extra-cellular molecules that are suspended in the watery solution.
Roughly ⅙ of the volume of a person""s body is made up of tissue gel, and it is essential to proper functioning of any type of soft tissue; among other things, it helps oxygen and nutrients reach cells, it aids in the removal of waste metabolites from tissue, and it helps tissue remain flexible and supple. Because proteoglycan filaments are so thin, molecules dissolved in tissue gel can permeate through the gel material with very little impedance; experiments have indicated that dye molecules can diffuse through tissue gel at rates of about 95 to 99 percent of their diffusion rates in water or saline.
Because nearly any type of soft tissue, in its normal and natural state, can be regarded as a type of hydrogel, many efforts have been made to create and use synthetic polymeric hydrogel materials as tissue implants. Most of these polymers are created by using non-parallel strands of long organic polymeric molecules (usually with chemical structures that are easier to work with and manipulate than glucosamine and glucuronate). Such molecules, to be suitable for use in a hydrogel, must be very hydrophilic (i.e., they must be able to attract and hold large quantities of water). This is most frequently accomplished by polymerizing precursor molecules that will provide large numbers of hydroxy groups (or other hydrophilic groups), on relatively short xe2x80x9cside chainsxe2x80x9d or xe2x80x9cside groupsxe2x80x9d that are bonded in a regular spaced manner to the long xe2x80x9cbackbonexe2x80x9d strands of the final polymer.
An example of a synthetic hydrogel of this nature is PHEMA (an acronym for poly-hydroxy-ethyl-meth-acrylate), which is used to make soft contact lenses, drug-releasing hydrogels, and similar articles. In contact lenses made of PHEMA, the polymer does not actually bend light. Instead, the water that dwells inside the PHEMA polymer when the lens is hydrated does that job. The hydrophilic PHEMA polymer merely holds water molecules together, in the shape of a contact lens. If a polymer such as PHEMA is dehydrated, it typically becomes brittle; as long as it remains filled with water, it stays soft and flexible. However, like most synthetic hydrogels, PHEMA does not have sufficient strength and durability to last for years (or decades) as a permanent surgical implant.
PHEMA is certainly not the only synthetic polymer used to create biocompatible hydrogels; other polymers that can swell and soften when saturated with water include various hydrophilic polyurethane compositions (e.g., Gorman et al 1998 and U.S. Pat. No. 4,424,305, Gould et al 1984), poly(vinyl alcohol) compositions (e.g., Wang et al 1999), and other compounds known to those skilled in this field of art.
The flexible, pliable, gel-like nature of a synthetic hydrogel (when saturated with water) arises mainly from crosslinking attachments between non-parallel fibers in the gel. Depending on the specific polymeric structure that has been chosen, these crosslinking attachments between the long xe2x80x9cbackbonexe2x80x9d chains in a polymer can be formed by covalent bonding, by hydrogen bonding or similar ionic attraction, or by entangling chains that have relatively long and/or xe2x80x9cgrabbyxe2x80x9d side-chains.
Regardless of which type of bonding or entangling method is used to bind the backbone chains together to form a hydrogel, the xe2x80x9ccouplingxe2x80x9d points between molecular chains can usually be flexed, rotated, and stretched.
In addition, it should be recognized that the backbone chains in hydrogel polymers are not straight; instead, because of various aspects of interatomic bonds, they are somewhat kinked, and can be stretched, in an elastic and springy manner, without breaking the bonds.
In a typical hydrogel, the fibers usually take up less than about 10% of the volume; indeed, many hydrogels contain less than 2% fiber volume, while interstitial spaces (i.e., the unoccupied spaces nestled among the three-dimensional network of fibers, which become filled with water when the gel is hydrated) usually make up at least 90 to 95% of the total volume. Accordingly, since the xe2x80x9ccouplingxe2x80x9d point between any two polymeric backbone chains can be rotated and flexed, and since any polymeric backbone molecule can be stretched without breaking it, a supple and resilient gel-like mechanical structure results when a synthetic hydrogel polymer is hydrated.
Various methods are known for creating conventional polymeric hydrogels. A number of such methods involve mixing together and reacting precursor materials (monomers, etc.) while they are suspended in water or other solvent. This step (i.e., reacting two or more monomers while they are suspended in a solvent) gives a desired density and three-dimensional structure to the resulting polymerized strands or fibers. The resulting material is then frozen, to preserve the desired three-dimensional structure of the fibers. The ice (or other frozen solvent) is then vaporized and removed, without going through a liquid stage, by a sublimizing process (also called lyophilizing), using high vacuum and low temperature. After the solvent has been removed, any final steps (such as a final crosslinking reaction and/or rinsing or washing steps, to remove any unreacted monomers, crosslinking agents, quenching agents, etc.) are carried out. The polymer is then gradually warmed up to room temperature, and it is subsequently saturated with water, to form a completed hydrogel.
These and other methods for creating synthetic polymeric hydrogels that are biocompatible and intended for surgical implantation are described in numerous patents, including U.S. Pat. Nos. 3,822,238 (Blair et al 1974), 4,107,121 (Stoy 1978), 4,192,827 (Mueller et al 1980), 4,424,305 (Gould et al 1984), 4,427,808 (Stol et al 1984), and 4,563,490 (Stol et al 1986). In addition, various methods of forming hydrogel coatings on the surfaces of other (xe2x80x9csubstratexe2x80x9d) materials are also described in various patents, such as U.S. Pat. Nos. 4,921,497 (Sulc et al 1990) and 5,688,855 (Stoy et al 1997).
There also have been efforts to reinforce hydrogels with an interpenetrating network (IPN) of fibers to enhance the soft hydrogel""s mechanical properties. These fiber reinforcements have been with either chopped or longitudinally aligned fibers within the hydrogel. A number of these efforts to develop xe2x80x9ccomposite hydrogelsxe2x80x9d apparently have focused on attempts to create synthetic pericardial tissue (i.e., the membrane that surrounds the heart); see, e.g., Blue et al 1991 and Walker et al 1991. Articles which describe these and other efforts to develop xe2x80x9ccompositexe2x80x9d hydrogels are discussed in two review articles, Corkhill et al 1989 and Ambrosio et al 1998. In addition, efforts to develop composite implants with fibers embedded in an xe2x80x9celastomeric matrixxe2x80x9d, for use in intervertebral discs designed for repairing spinal damage, are described in various patents such as U.S. Pat. No. 4,911,718 (Lee et al 1990) and U.S. Pat. No. 5,171,281 (Parsons et al 1992).
U.S. Pat. No. 5,855,610 (Vacanti et al 1999) also describes an approach to implanting under the skin, in a first location, a flexible porous network that has been seeded with cells, then leaving it there for a number of days to encourage fibrous tissue growth into it, and then removing it and implanting it into a second location. Although that approach is quite different from the subject invention, that patent is worth noting because it contains a very extensive listing of patents and scientific articles in the field of flexible implants.
As used herein, all references to xe2x80x9cimplantsxe2x80x9d or xe2x80x9cimplantationxe2x80x9d (and all terms such as surgery, surgical, operation, etc.) refer to surgical or arthroscopic implantation of a reinforced hydrogel device, as disclosed herein, into a mammalian body or limb, such as in a human patient. Arthroscopic methods are regarded herein as a subset of surgical methods, and any reference to surgery, surgical, etc., includes arthroscopic methods and devices. The term xe2x80x9cminimally invasivexe2x80x9d is also used occasionally herein, even though it is imprecise; one should assume that any surgical operation will be done in a manner that is minimally invasive, in view of the needs of the patient and the goals of the surgeon.
Despite all the efforts cited above (and numerous others in that field, as well), surgically implantable hydrogels that are intended as permanent prosthetic replacements for damaged or diseased tissue suffer from a number of important limitations, including (1) relatively low strength and durability, and (2) difficulties in anchoring them permanently in a desired location, in ways that provide adequate strength. Both of these crucial factors severely limit the number and variety of uses for such hydrogels that have been developed and commercialized to date. In general, they are used today mainly for disposable external use (such as in contact lenses, and in skin patches), and for resorbable devices that will release a desired drug for a prolonged period and then gradually dissolve and disappear inside the body.
By contrast, this primary goal of this invention is to disclose a hydrogel device with a reinforcing mesh embedded in the hydrogel, to help makes the gel device strong enough and durable enough to be surgically implanted in a knee joint, as a replacement for a damaged meniscus.
Meniscal Tissues in Knees
Each knee joint of a human contains a xe2x80x9cmedialxe2x80x9d meniscus, and a xe2x80x9clateralxe2x80x9d meniscus. The lateral meniscus is located on the outer side of the leg, directly above the location where the upper end of the fibula bone is coupled to the tibia (xe2x80x9cshinbonexe2x80x9d). The medial meniscus is located on the inner side of the leg.
Each meniscus (also referred to, especially in older texts, as a xe2x80x9csemilunar fibrocartilagexe2x80x9d) has a wedged shape, somewhat comparable to a segment from an orange or other citric fruit, but with a substantially larger curvature and xe2x80x9carcxe2x80x9d. The thickest region is around the periphery (which can also be called the circumference, the rim, and similar terms). When implanted into a knee, this peripheral rim normally will be anchored to the surrounding wall of a fibrous xe2x80x9ccapsulexe2x80x9d which encloses the knee joint and holds in the synovial fluid, which lubricates the cartilage surfaces in the knee. The two ends of each semi-circular wedge are coupled, via ligaments, to the xe2x80x9cspinexe2x80x9d protrusions in the center of the tibial plateau.
The inner edge of a meniscus is the thinnest portion of the wedge; this edge can also be called the apex, the margin, and similar terms. It is not anchored; instead, as the person walks or runs, each meniscus in a knee is somewhat free to move, as it is squeezed between the tibial plateau (beneath it) and a femoral runner (above it). The bottom surface of each meniscus is relatively flat, so it can ride in a relatively stable manner on top of the tibial plateau. The top surface is concave, so it can provide better, more closely conforming support to the rounded edge of the femoral runner. Because of its shape, location, and ability to flex and move somewhat as it is pushed, each meniscus helps support and stabilize the outer edge of a femoral runner, as the femoral runner presses, slides, and xe2x80x9carticulatesxe2x80x9d against the portion of the tibial plateau beneath it.
However, because all four of the menisci inside a person""s knees are in high-stress locations, and are subjected to frequently-repeated combinations of compression and tension (and sometimes abrasion as well, especially in people suffering from arthritis or other forms of cartilage damage), meniscal damage often occurs in the knees of humans, and occasionally other large animals.
It should also be noted that, in humans, meniscal-type tissues also exist in shoulder joints and wrist joints.
Various efforts have been made, using prior technology, to repair or replace damaged meniscal tissue. However, because of the complex structures and anchoring involved, and because of the need to create and sustain extremely smooth and constantly wet surfaces on the inner portions of each meniscal wedge, prior methods of replacing or repairing damaged meniscal are not entirely adequate.
Accordingly, one object of this invention is to disclose a hydrogel device, with a reinforcing mesh that helps to make the gel component strong enough and durable enough to be surgically implanted in a knee joint, as a replacement for a damaged meniscus.
Another object of this invention is to disclose a composite meniscal implant having a hydrogel component, with a reinforcing mesh that is: (i) exposed on the outer peripheral surface of the implant, to enable secure anchoring of the meniscal implant around the periphery; and, (ii) hidden and internal on the inner wedge surfaces of the meniscus, so that the hydrogel coating layers will remain completely smooth and will not abrade the cartilage surfaces on the femoral runner and tibial plateau that will rub and articulate against the meniscus.
Another object of this invention is to disclose a composite implant device having a hydrogel component reinforced by a three-dimensional mesh which creates a reinforcing xe2x80x9cinterpenetrating networkxe2x80x9d that resembles certain types of natural body tissues, such as interfaces between bone and cartilage.
Another object of this invention is to disclose a surgical implant having a hydrogel component that partially encloses a three-dimensional mesh, and wherein the mesh emerges from one or more selected locations in the implant, to provide improved anchoring capabilities, but wherein the mesh is not exposed on certain other surfaces of the implant, so that a very smooth hydrogel surface will cover those portions of the implant.
Another object of this invention is to disclose a wedgeshaped implant having a smooth inner edge and an outer rim designed for anchoring, which might be useful in some situations where a surgeon must repair a damaged or disease joint other than a knee, such as a shoulder, wrist, ankle or elbow joint, or in an interface between two bones in another part of the body, such as a hand or foot.
These and other objects of the invention will become more apparent through the following summary, drawings, and description of the preferred embodiments.
A device designed for surgical implantation to replace damaged tissue (such as a meniscus in a knee) is disclosed, having a hydrogel component reinforced by a three-dimensional mesh. The mesh component provides strength and structural support for the implant, which has at least one articulating surface, and at least one anchoring surface. In one embodiment, the mesh emerges from one or more selected locations around the peripheral rim of a meniscal implant, to provide anchoring attachments that can be sutured, pinned, clipped, or otherwise securely affixed to the fibrous capsule that surrounds the knee. Preferably, the rim surface should be porous, to promote scar tissue (or, in some cases, bone tissue) ingrowth into the implant, to create a strong permanent anchoring support for the implant. In addition, at least some portion of the mesh component preferably should extend through most of the thickness of the hydrogel portion, to create a reinforcing xe2x80x9cinterpenetrating networkxe2x80x9d (IPN) of fibers, modelled after certain types of natural body tissues. The xe2x80x9carticulatingxe2x80x9d surfaces of a meniscal wedge, which will rub and slide against femoral and tibial cartilage, should be coated with a hydrogel layer which is smooth and nonabrasive, and made of a material that remains constantly wet. This composite structure, with hydrogel layers surrounding an embedded mesh component, provides a joint-repair implant with improved anchoring, strength, and performance compared to implants of the prior art. Because of certain design advantages, this type of implant may also be useful in surgical repair of other joints, such as damaged shoulders, wrists, ankles or elbows, or in surgical repair of feet or hands.